High voltage alternating current (HVAC) cable systems comprise a conductor located at the centre of the cable, for delivering an electric current. The conductor is usually surrounded by plurality of layers arranged radially outside of the conductor, where each layer has a specific function. A first layer arranged over the conductor may comprise cross-linked polyethylene (XLPE), providing electrical insulation.
Arranged radially outside of the XLPE layer there may be provided a metallic sheath, for electrically shielding and protecting the conductor from abrasion, corrosion and moisture. The current flowing through the conductor induces a voltage in the metallic sheath surrounding the conductor, and if the sheath forms a closed circuit, a circulating current will be generated in the metallic sheath. This circulating current results in power loss and generation of heat in the metallic sheath, causing the temperature of the cable system to increase.
An increase of temperature can lead the cable insulation to deteriorate. Therefore, the amount of current a cable can carry before deteriorating is restricted by a temperature rating. Commonly, the maximum allowable temperature of a HVAC conductor is 90° Celsius for XLPE insulation. The term ampacity is used to describe the conductor's capacity to carry current whilst staying within its temperature rating.
The ampacity of the cable will be dependent on a number of different factors such as the ambient temperature, the electrical current in the cable, the electrical resistance of the cable and the heat dissipating properties of the surroundings. The electrical current is a fixed requirement, specified by how much power the cable needs to deliver, and it is usually not an option to reduce this parameter. The ambient temperature and heat dissipating properties of the surroundings are often hard to affect, and the other specifications of the cable must therefore be adapted to these relatively inflexible factors to be able to deliver the required amount of electric current without exceeding its ampacity.
High voltage cable systems are often required to traverse stretches of water, resulting in relatively large variations in the heat dissipating properties of the cable's surroundings. These submarine cables are typically buried at shallow soil depths on the bottom of the seafloor, where water penetrates the soil surrounding the cable. The water provides the surroundings of the submarine cable with relatively favourable heat dissipating properties, dissipating heat far better than e.g. air and other types of soil. Parts of the submarine cable can also rest on the seafloor, or stretch through a body of water. In these cases, the environment of the submarine cable will have as good, if not better heat dissipating properties compared to being buried below the seafloor.
When the cable is brought ashore, it may either pass an underground region where the soil is not saturated with water, it may be brought along a surface of the ground, it may be provided elevated on a frame in the air or it may pass through a submarine region with inferior heat dissipating properties. Typically, if the cable passes an underground region, it may lie inside a horizontal directional drilling (HDD) pipe which has been provided in this region. In all these cases, the landfall region will usually have reduced heat dissipating properties, resulting in a so-called thermal bottleneck. To retain the same ampacity of the cable in the thermal bottleneck as in the submarine region, it is necessary to adapt certain specifications of the cable system.
Previous solutions to this bottleneck problem have been to provide a large increase of the cross sectional area of the conductor in the landfall region, giving a decrease in the electrical resistance of the conductor. However, due to the tendency of an alternating electric current (AC) to become distributed within a conductor such that the current density is largest near the surface of the conductor, the cross sectional area must be drastically increased if the ampacity is to be maintained without exceeding set thermal restrictions. Increasing the cross sectional area of the conductor has many negative effects: it requires substantially more material, such as copper, leading to an increase in production costs. Often the increase of the cross sectional area is so large that production is not practically possible. An increased cross sectional area of the conductor also leads to significantly higher sheath losses. Handling a submarine cable with a large cross sectional area also presents a multitude of problems due to added weight and size of the cable, requiring corresponding dimensioning of systems on board a cable-laying vessel.
WO2016/082860 discloses a cable with conductor sections with different cross-sectional layout that are thermally joined. The cable comprises a conductor at its centre, and may comprise a semiconductor layers around the conductor and one continuous outer sheath made of thermoplastic or thermosetting polymer enclosing the semiconductor layers. The cable may also comprise additional metallic sheaths. The prior art partially solves the problem of having a conductor with a large cross sectional area along the entire cable, as only the section of the cable located in the thermal bottleneck is fitted with a conductor with a larger cross sectional area. However, this still leaves a substantial section of the cable system with a conductor of an undesirably large cross sectional area.